8 



T, 



■q U. S. DEPARTMENT OF AGRICULTURE, 

BUREAU OF PLANT INDUSTRY— BULLETIN NO. 173. 

B. T. GALLOWAY, Chief of Bureau. 



SEASONAL NITRIFICATION AS INFLUENCED 
BY CROPS AND TILLAGE. 



C. A. JENSEN, Agriculturist. 



Issued April LS, 1910. 




WASHINGTON: 

GOVERNMENT PRINTING OFFICE. 
1910. 



U. S. DEPARTMENT OF AGRICULTURE, 

BUREAU OF PLANT INDUSTRY— BULLETIN NO. 173. 

B. T. GALLOWAY, Chief of Bureau. 



SEASONAL NITRIFICATION AS INFLUENCED 
BY CROPS AND TILLAGE. 



C. A. JENSEN, Agriculturist. 



Issued April 18. 1910. 




WASHINGTON: 

GOVERNMENT PRINTING OFFICE. 

1910. 



^ 



^\ 






BUREAU OF PLANT INDUSTRY. 



Chief of Bureau, Beverly T. Galloway. 
Assistant Chief of Bureau, O. Harold Towell. 
Editor, J. E. Rockwell. 
Chief Clerk, James E. Jones. 



Dry-Land Agriculture Investigations. 

scientific staff. 

E. C. Chilcott, Agriculturist in Charge. 

J. S. Cole, Expert. 

J . M. Stephens, Special Agent. 

W. W. Burr, E. F. Chilcott, O. J. Grace, F. I>. Kennard, and J. E. I'ayne, Assistants. 

A. L. Hallsted, Clarence Plath, H. R. Reed, and John Thysel, Assistants 

Physical IjAuoratory. 

scientific staff. 

I.yiuaii J. Briggs, Physicist iti Charge. 

J. O. Belz, Assistant. 

J. W. McLane and Julia R. Pearce, Laboratory Assistants. 
173 

2 



MAY 2 1910 






LETTER OF TRANSMITTAL. 



U. S. Department of Agriculture, 

Bureau of Plant Industry, 

Office of the Chief, 

Washington, D. C, December 23, 1909. 

Sir: I have the honor to transmit here\\ath and to recommend for 
pubhcation as Bulletin No. 173 of the series of this Bureau the 
accompanj'jng manuscript, entitled ''Seasonal Nitrification as Influ- 
enced by Crops and Tillage," by Mr. Charles A. Jensen, Agriculturist, 
of this Bureau. 

The Office of Dry-Land Agriculture, of this Bureau, is carrying 
out extensive investigations in crop rotation and cultivation methods 
at thirteen stations located at widely different points in the Great 
Plains area. The purpose of these investigations is to ascertain the 
best methods of agriculture in these semiarid regions. To do this 
intelligently it is necessary not only to study the immediate effect 
of these methods on crop yields, but also to study the resultant 
changes occurring in the physiological properties of the plant nutri- 
tive solution, which is recognized as one of the fundamental factors 
in crop yields. It is therefore necessary to supplement the field 
work with laboratory investigations having for their aim the solu- 
tion of the problems presented by the crop returns from the rotation 
systems mentioned. 

The accompanying manuscript shows the seasonal changes in the 
water-soluble nitrates in summer-fallowed land, wheat land, and corn 
land and the rates and extent of the seasonal removal of the nitrates 
by these crops. The investigations were made by Mr. Jensen wliile 
he was superintendent of the Bellefourche substation of the Office 
of Dry-Land Agriculture. The work was conducted in cooperation 
with the Physical Laboratory of this Bureau, which has general 
supervision over the physical measurements carried out at the dry-land 
stations of the Bureau. 

The results described in the paper are, I believe, sufficiently signifi- 
cant to demonstrate the importance of a more extended development 
of this line of investigation at the stations. 
Respectfully, 

B. T. Galloway, 

Chief of Bureau. 

Hon. James Wilson, 

Secretary of Agriculture. 

173 

3 



CONTENTS 



Page. 

Introduction 7 

Description of the soil in which the experiments were conducted ' 8 

Field history of the plats 9 

Methods of making determinations 9 

Discussion of results of determinations 12 

Nitrates in the fallow plat 12 

Nitrates in the spring-wheat plat 14 

Comparison of nitrates in fallow, spring-wheat, and corn plats IG 

Withdrawal of nitrates by corn roots 17 

Relation of nitrification to field factors 18 

The disappearance of nitrates 21 

Denitrification 22 

Seasonal variations in nitrification 23 

Change of water-soluble nitrates to albuminoid nitrogen 25 

Relation of nitrification to summer fallow 26 

Summary 27 

Index 29 

173 

5 



ILLUSTRATIONS. 

Page. 
Fig. 1. Diagram showing the parts per million of water-soluble nitrates in dry 
soil in the fallow plat in each 6-inch soil layer to a depth of 2 feet; 
also precipitation curve 12 

2. Diagram showing the parts per million of water-soluble nitrates in dry 

soil in the spring- wheat plat in each 6-inch soil layer to a depth of 2 

feet; also weekly increase of wheat plants 14 

3. Diagram showing the average parts per million of water-soluble nitrates 

in dry soil to a depth of 2 feet in the summer-fallow, spring-wheat, 
and corn plats; also weekly increase of wheat plants and precipita- 
tion curve 16 

4. Diagram showing the parts per million of water-soluble nitrates in dry 

soil in each 6-inch soil layer in the corn plat to a depth of 2 feet 17 

5. Diagram showing the average parts per million of water-soluble nitrates 

in dry soil in the fallow plat to a depth of 2 feet; also soil-moisture 
curve and precipitation curve 19 

6. Diagram showing the average parts per million of water-soluble nitrates 

in dry soil to a depth of 2 feet in the spring- wheat plat; also soil- 
moisture, precipitation, and weekly crop-increase curves 20 

7. Diagram showing the time and location of the maximum amount of 

nitrates in the wheat and fallow plats 21 

17.3 
6 



B. P. I.— 536. 



SEASONAL NITRIFICATION AS INFLUENCED 
BY CROPS AND TILLAGE. 



INTRODTJ.CTION. 

The attempt to bring the vast areas of prau"ie lands of the Great 
Plains under a profitable system of agricidture has been undertaken 
with renewed vigor during tlie last few years, both by the older settlers 
and by man}' new ones entering the Plains. These new settlers come 
mainly from the Middle West, where the climatic and soil conditions 
are very different from those in the newer areas where they settle. 
On the Great Plains the rainfall is likely to be too little rather than 
too much for good crop returns and, moreover, it does not always 
come at the time most convenient for the farmer; besides, the wind 
movement, especially in the spring, when the crops are being planted 
and when the surface of the soil is usually exposed in a rough con- 
dition, saps the soil of its moisture very rapidly. 

Under such conditions it becomes necessary to control the quantity 
and the movement of the soil moisture, and as this moisture consti- 
tutes the plant nutritive solution it is obvious that not alone the 
absolute amounts of soil moisture present but the nutritive character 
of this solution become of importance, so that any cultural system 
inaugurated for the purposes of moisture conservation should be such 
that the quality as well as the quantity of the nutritive solution can 
be controlled. 

It has often been observed that of two contiguous })lats on the 
same type of soil one may yield considerably higher than the other, 
although the amount of soil moisture may be the same in both. In 
such cases it is often found that the properties of the nutritive solu- 
tions from the two plats are markedly different m their behavior 
toward plant growth, and often the history of the plats will show that 
the cultural methods have been different and that these may explain 
the differences found in the character of the soil moisture constituting 
the plant nutritive solution. The mere storing up, then, of soil mois- 
ture is not always sufficient for the purposes of good crop returns. 

Durmg the last few years a number of substations have been estab- 
lished by the Bureau of Plant Industry tliroughout the Great Plains 
173 7 



8 SEASONAL NITRIFICATION AS INFLUENCED BY CROPS, ETC. 

area for the purpose of finding out, if possible, the best methods of 
handlmg the prairie lands there so that profitable crop returns might 
be realized. At all of these stations a large number of rotations have 
been established, and other plats have been set aside for experiments 
with various methods of cultivation without regard to crop rotations. 

In the spring of 1907 the writer was put in charge of the substation 
at Belief ourche, S. Dak., where experiments in dry-land agriculture 
were inaugurated. The station site was on virgin prairie lands, so 
nothing was done that year beyond breaking up and preparing land 
for use in 1908. In the spring of that year an elaborate series of 
rotation systems was established, and a large number of other plats 
were set aside for use in the studies of soil-moisture conservation by 
different methods of cultivation, the success of the methods being 
measured finally l)y the crop yields, though moisture determinations 
were made (hiring the entire season. 

The work to be described was done on some of the plats used for 
the purpose of trying out different cultural methods, it being thought 
that different crops and different tillage methods woidd influence 
the nutritive properties of the soil solution. Owing to the press of 
field duties and to the inconvenient location of the station as regards 
supplies, it was found necessary to limit the laboratory work to a 
study of the natural activity of nitrification in the soil as measured by 
determinations of nitrates, nitrites, and ammonia in three plats receiv- 
ing different treatments and bearing different crops. 

The literature on nitrifying organisms and on nitrification is very 
voluminous, and while the author does not by any means claim to 
have read it all, it is believed that most references which have any 
direct bearing on the present subject matter have been cited. In no 
case was there found reported work on nitrification in the field as 
influenced by crops and tillage that had been carried on continu- 
ously throughout the crop season. The few references found which 
might have a bearing were generally reports on work carried on in 
humid regions or in the laboratory, the conclusions of which could 
hardly be extended to the semiarid regions. Much of the literature, 
however, was helpful in formulating suggestions explaining the 
phenomena found. 

DESCRIPTION OF THE SOIL IN WHICH THE EXPERIMENTS WERE 

CONDUCTED. 

The soil of the station and of the surrounding country is known as 
"gumbo" and is a heavy, black or gray, clay loam to a depth of 2 to 
3 feet, and below this depth hea^'y clay usually occurs, extending to 
6 feet or deeper, overlying the partially disintegrated cretaceous shale 
which is the geological formation of the whole surrounding country. 

173 



METHODS OF MAKING DETERMINATIONS. U 

FIELD HISTORY OF THE PLATS. 

The prairie sod, covered origin;dl\' with western wheat-grass 
{Agropyron occidentale) , bullalo grass (Bulhilis dactyloides), and 
gramma (Bouteloua oligostachya) , witli minor quantities of sedges 
(Car ex JiH foil a and C. stcnopJiyUa) and other grasses, together with a 
few vetches, w^as broken in the spring of 1907 to a deptli of about 3 to 
4 inches, backset (replowed) in the fall of 1907 to a depth of about 5 
mches, and left open until the spring of 1908, when it was disked and 
harrowed until a good seed bed was secured. Such was the treat- 
ment of the spring-wheat plat and the corn plat used in the work. A 
plat reserved for summer-fallow used in this work was spring-plowed 
to a depth of about 8 inches, smoothed with a disk, and harrowed. 
This plat was cultivated during the summer to keep weeds down and 
to conserve the moisture. 

The determinations on the corn plat were not begun as early as on 
the wheat and fallow plats. As this plat was not planted as early as 
the wheat plat, it was taken for granted that the soil conditions w^ould 
be the same as those of the fallow plat until the time of planting the 
corn. 

METHODS OF MAKING DETERMINATIONS. 

The chemical methods used were those described in Bulletin No. 31 
of the Bureau of Soils. The soil samples were collected in duplicate 
from 6-inch layers to a depth of 2 feet — that is, in to 6, 6 to 12, 12 to 
18, and 18 to 24 inch depths — and the two cores from each soil layer 
made into a composite sample. The samples as bored up with the 
auger were transferred to air-tight tin cans, taken immediately to the 
laboratory, weighed, dried in an oven at a temperature of about 100° 
C, and again weighed. In preparing the solution it was found 
necessary to adopt this drying method in order to insure thorough 
mixing of the soil and the w^ater, as it was jDractically impossible to 
break dow^l the moist gumbo pieces directly. The ratio of soil to 
wate^ was 1 to 5 (100 grams of soil to 500 c. c. of water). After 
drymg, the soil particles readily broke down, and after thorough 
stirring in the water would settle out quite clear in about 20 to 30 
minutes. The supernatant liquid was then poured into Chamberland 
filters and filtered. Such a procedure of drying the soil could, of 
course, not be employed were determinations to be made of such soil 
elements as are derived directly from the soil minerals, as potassium, 
phosphoric acid, etc., but owing to the origin and nature of the 
nitrates and nitrites in the soil the solubility is not increased ; neither 
is it likely that decomposition of a disturbing nature would take place. 
Experiments w^ere made to compare the two methods of obtaining the 
24493— Bui. 173—10 2 



10 SEASONAL NITRIFICATION AS INFLUENCED BY CROPS, ETC. 

soil solutions from soil that was first dried and from undried sami)les 
direct from the field. The results aj^jreed within the limits of experi- 
mental error, and as better mixing was possible by using the dried 
samples this procedure was an advantage. 

The determination of nitrates under ideal conditions does not 
present any serious diliiculty if the procedure is carefully followed 
out and precaution against errors observed. As is well known, 
however, difliculties are likely to be encountered when nitrates 
are to be determined in soil extracts, es])ecially if the extract 
is highly colored w^ith soluble organic matter or if it contains con- 
siderable soluble inorganic salts, especially chlorids. There are thus 
three main sources of error in nitrate determination in a soil extract : 
Foreign organic color, which is superimposed on the yellow color 
developed on nitration of the phenoldisul])honic acid, which renders 
the colorimetric reading more dillicult; the soluble organic matter 
may be acted upon by the nitrates, or possibly by the acid, thus 
introducing a chemical error; the presence of considerable amounts of 
alkali salts, of which chlorids seem to be the most serious ones. 

The soil extract obtained in the work here reported did not contain 
enough chlorids to cause error, as the amounts were too small to 
measure accurately by means of titration; only traces were ever 
observed, and frequently none at all. 

The errors which might be introduced because of the presence of 
soluble organic matter are believed to be too small to seriously con- 
sider, as the only soil samples which gave appreciable organic colors 
were those from the fallow plat, and no serious difficulty was en- 
countered in making the colorimetric readings of these extracts, the 
organic color being of quite a different shade from the ^^ellow color 
due to nitration. The error which may have been introduced 
through the chemical disturbance of the organic matter was neglected, 
it being considered too small to be of serious consequence; as stated 
above, the organic color was not strong. All points considered, it was 
decided that under the circumstances and conditions less error would 
be involved in using the solutions directly as obtained than by trying 
t(^ free them of organic matter by oxidation or reduction. 

It should be kept in mind that the chief value of the results 
obtained lies in their relativity more than in the absolute 
amounts found. This weekly relation of the amounts of nitrates in 
the various soil layers in the various plats could not be seriously 
disturbed by any slight error which might possibly have been intro- 
duced because of the presence of soluble organic matter in the soil 
extracts. 

It has been quite well made out that no appreciable error was 
introduced in quickly drying the soil samples in the oven at 100° C. 

173 



METHODS OF MAKING DETEKMINATIONS. 



11 



This was established at the time and has been also found in the 
bacteriological laboratory of this Bureau to be a safe procedure for 
purposes of comparison. 

A comparison of results obtained by the various metliods of extrac- 
tion is given in Table I. 

Table I. — Nitrites, nitrates, and ainvionia found in dry soil treated iti various trays. 



Soil treatment. 



Fresh iml reatecl soil, using undisf illed water 

Fresh untreated soil, using distilled water 

Fresh untreated soil plus ehloroforni, using undistilled water 
Fresh untreated soil plus chloroform, using distilled water. . . 
Soil dried in oven at 100° C 



Parts per million in dry soil. 



Nitrites 


Ammonia 


Nitrates 


(NO2). 


(NH3). 


(NO3). 


0..55 


1.5 


26.7 


..•)0 


.8 


22.2 


..5.5 


2.5 


22.2 


.55 


2.0 


25.0 


.55 


3.33 


36.4 



The oven-dried sample in this case gave a higher proportion of 
ammonia and nitrates than the other samples. It is possible, though, 
that nitrification went on to some extent before the sample was prop- 
erly dried, as the gasoline drying oven was troublesome, this being 
its first use. The soil sample was in the oven most of the day before 
the trouble with the burner was located, and it was warmed most 
of the day without quickly drying. 

The following set was run through the next day, a soil behig used 
that had been carefully prepared for a winter-wheat seed bed the 
previous year but which had not been planted : 



Table II. 



-Nitrites, nitrates, and ammonia found in dry soil from a seed bed treated in 
various ways. 







Parts per 


million in dry soil. 


Soil treatment. 


Nitrites 

(NOs), 


Ammonia 
(NH3). 


Nitrates 

(NO3). 


Fresh tmtreated soil immediatelv extracted.. 


0.28 
.28 

.20 

.33 




133.3 


Fresh soil plus chloroform and immediately extracted 




1.33.3 


Fresh soil after three hours' treatment with chloroform and dried quickly 
at 103° C 


133. 3 


Fresh soil dried immediately in oven without being treated 
form . .... 


with chloro- 


133. 3 



The ammonia was not read, as the solution contained foreign color- 
ing matter. These and the previous results give an idea of the limits 
of experimental error. The standard solutions used for comparison 
in the colorimeter were made up fresh on the day they were used, 
and all readings were made within a few hours. The nitrite reagents 
were kept separate and small portions mixed as needed for use. 

The systematic determinations were begun April 27, and from then 
until July 31, about ten days after harvest, the determinations were 
made once a week. 

173 



12 SEASONAL NITRIFICATION AS INFLUENCED BY CROPS, ETC. 



DISCUSSION OF RESULTS OF DETERMINATIONS. 



NITRATES IN THE FALLOW PLAT. 

Figure 1 shows the parts per inilhon of nitrates calculated for the 
oven-dried soil in each 6-inch layer in the fallow plat from April 27 
to July 31 and the precipitation for the same period. About 1 inch 
of rain had fallen from April 1 to April 27. The weekly periods are 
laid off on the abscissa, and the parts per million of nitrates on the 
ordinate. The precipitation scale is on the right-hand side of the 
figure. 

The first two sets of determinations show decreasing amounts of 
nitrates from the surface 6-inch layer downward, the second foot con- 



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Fig. 1. — Diagram showing the parts per milhon of water-soluble nitrates in dry soil in the fallow plat in 
each C-inch soil layer to a depth of 2 feet; also precipitation curve. 

taining very small amounts. There were no marked changes until 
May 9, when the surface 6 inches of soil showed a considerable increase 
in nitrates, and it is also seen that at this time this soil layer contained 
a maximum amount of water-soluble nitrates. The nitrates in the 
second layer, 6 to 12 inches, had more than doubled since the week 
before, but there were no considerable changes in the lowest two 
layers. By the next week, May 16, the surface 6 inches of soil had 
decreased about forty parts per million, and there was also a decrease 
in the 6 to 12 inch layer. By the next week, on May 22, a marked 
increase in the amount of nitrates is evident in the 12-inch layer, it 
having increased about seventy parts per million during six days, 
and the surface 6-incli layer had decreased about ten parts per million. 
At this date the 12-inch layer contained the maximum quantity of 

173 



DISCUSSION OF RESULTS OF DETEEMINATTONS. 13 

nitrates. Up to this time tliere were no great changes in the nitrates 
in the 18 and 24 inch hiyers, but from May 22 to June 5 these laj^ers 
accumuUited considerable amounts, and on the hitter date the 18-inch 
layer contained the maximum, which remained constant until the 
next week, June 12. In the mean time, the 24-inch layer accumulated 
considerable quantities of nitrates, reaching its maximum on June 12. 

It will be noticed in general that from April 27 to June 5 the 
maximum amount of water-soluble nitrates shifted every two weeks 
to the next lower layer and that once any layer had accumulated 
a maximum it decreased and in general never again contained as 
large amounts as the maximum of the next lower layer, which was 
reached at a later date. 

From June 12 to June 19 there was an interesting change, in that 
every soil layer wdiich had recently been actively accumulating 
nitrates suddenly showed a marked decrease in amounts of water- 
soluble nitrates, and all of the layers on June 19 contained nearly 
the same amount of nitrates. The upper 6-inch layer did not show 
any such decrease, the reason for which will be discussed later on. 
From June 19 till July 31 there were no regular changes, all of the 
curves zigzagging irregularly. 

In view of the marked increases in the amount of nitrates in the 
18 to 24 inch layer, it is to be regretted that determinations were not 
made in the third foot also, but time did not permit the increased 
work. The 18 to 24 inch layer never contained a maximum amount 
of nitrates in comparison with the other layers, but the results cer- 
tainly indicate that nitrification is quite active at a depth below 2 
feet in these prairie soils when they are brought under cultivation. 
Bazarewski" found very few nitrifj'ing bacteria, however, at a depth 
of 50 centimeters and reports that they were "plentiful at a depth 
of 10 centimeters." It w^ill be noticed from figure 1 that the 18 to 24 
inch soil layer accumulated as much water-soluble nitrate as did the 
to 6 inch layer, so nitrifying organisms are presumably active in 
this prairie soil to a depth considerably greater than 2 feet. Hunt ^ 
likewise reports that nitrification took place to but a small extent 
below a depth of 2 feet, and the figures given by him show that under 
the conditions discussed nitrification was not very active in even 
the second foot. 

In this connection it may be of interest to mention that the un- 
broken prairie "gumbo" never becomes moistened deeper than 
about 8 to 10 inches even after the heaviest rains. Cultivation, 

a Bazarewski, L. von. Nitrification and Denitrification in Soils. Reviewed in Ex- 
periment Station Record, vol. 21, no. 1, July, 1909, pp. 20-21. 

b Hunt, T. F. The Importance of Nitrogen in the Growth of Plants. Bulletin 247, 
New York (Cornell) Agricultural Experiment Station. 
173 



14 SEASONAL, NTTRIFICATIOlSr AS INFLUENCED BY CROPS, ETC. 

however, soon results in the soil becoming moistened deeper than 
6 feet within one year after breaking the sod and cultivating the 
soil. It is fair to assume, therefore, that nitrification in the un- 
broken land can not take place to any great extent much below a 
depth of 1 foot. 

NITRATES IN THE SPRING-WHEAT PLAT. 

Figure 2 shows the results obtained from determinations made 
on the spring-wheat plat. These determinations were not begun 
until May 9, but a vertical section made in figure 1 on May 9 will 
show that the order is about the same at this point on both figures. 

Figure 2 does not contain the precipitation curve, but in addition 
to the curves showing the amounts of nitrates in the 6-inch soil Inyers, 



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J<'IG. 2. — Diagram showing the parts per million of water-soluble nitrates in dry soil iu the spring-wheat 
plat in each 6-inch soil layer to a depth of 2 feet; also weekly increase of wheat plants. 

it shows the curve representing the weekl}' increase in the dry weight 
of the wheat plants. The increase was determined each week by 
cutting four 3-foot squares, two near -each end of the 2 by 8 rod plat, 
both green and air-dry weights being obtained. Only the dry 
weights are platted in the figure. 

A comparison of figure 1 and figure 2 shows the same general fact, 
namely, that the maximum amount of water-soluble nitrates in any 
soil layer is shifted every two weeks to the next lower layer. The 
detailed variations are about the same in the two plats, but there is 
one general difference — the date of the maximum- accumulation of 
nitrates in the respective soil layers in the two plats is shifted one 
week forward in the case of the spring-wheat i)lat — that is, each of 
the 6, 12, and 18 inch layers, respectively, attained its maximum 

173 



DISCUSSION OF RESULTS OP DETERMINATIONS. 15 

accumulation of water-soluble nitrates just one week later in the 
wheat plat than in the fallow plat. It will be remembered that it 
was stated that the fallow plat was plowed to a depth of about 8 inches 
in the spring of 1908 and that the wheat plat had not been plowed 
since being backset in the previous fall to about 5 inches deep. There 
was thus a difference in the mechanical condition of the two plats; the 
summer-fallow plat being more open and stirred to a greater depth, 
there was presumably a more rapid access of air, and in consequence 
a more rapid rise in temperature. Hunt °' also found that earl}^ ])low- 
ing promoted nitrification more rapidly than late ])lowing, and pre- 
sumably early plowing would promote nitrification more rapidly than 
no plowing at all. 

It was expected that some information would be obtained in regard 
to the rate at which the growing wheat plants removed the nitrates 
from the different soil layers, that being one of the main points 
sought for in the work. As a matter of fact, it will be noticed that 
the curves of the two figures are so much alike in the general trend 
that were they not scaled or labeled in any way it would be difficult 
to say which figure represented the fallow plat and which the wheat 
plat. The shifting of the dates of the maximum accumulation of 
nitrates in the individual soil layers one week back in the case of the 
fallow plat might be explained by the difference in the tillage. By 
using the fallow plat as a check on the wheat plat, which is realh^ 
what it is, it will be noticed that aside from this shifting already 
mentioned the only general differences are the smaller amounts of 
nitrates found in the wheat plat and that after June 19, when the 
general decrease in nitrates took place — as in the fallow plat — the 
nitrates in each soil layer in the wheat plat were more constant in 
both total amounts and amplitude of variation. Had the results 
on the fallow plat not been obtained as a check on the results ob- 
tained on the wheat plat, it might have been supposed that during 
the time immediately following June 12, when the wheat plants 
were making rapid increase in growth, the crop was removing nitrates 
much more rapidly than they were being formed. Even thus one 
would hardly expect the nitrates to be removed so spontaneously and 
almost completely from each 6-inch soil layer. One can not, then, 
from the amounts of water-soluble nitrates found, determine the soil 
layers in which the wheat roots were feeding most rapidly or demon- 
strate any progressive downward ranging of the roots as the season 
advanced. The apparent ''removal" of the nitrates from the soil 
is due, then, to some other factor than the presence of the crop. 

Even after June 19, when the marked decrease in the amounts of 
water-soluble nitrates occurred, there is no consistent correlation 

a Hunt, T. F.,loc.cit. 
173 



16 SEASONAL NITRIFICATION AS INFLUENCED BY CEOPS, ETC. 

between the weekly increase in the dry weight of the wheat crop and 
the amounts of nitrates present. On the contrary, from June 24 to 
July 3 there was a general increase in the amounts of nitrates corre- 
sponding to the greatest weekly increase in the growing crop, 

COMPARISON OF NITRATES IN FALLOW, SPRING-WHEAT, AND CORN 

PLATS. 

Figure 3 shows the amounts of nitrates — averages of the four 6- 
inch soil layers — found during the season in the wheat plat, the fal- 
low plat, and the corn plat; also the precipitation curve and the 
weekly crop-increase curve. It is to be regretted' that the same 
amount of data could not have been obtained from the corn plat as 
from the wheat plat, but time did not permit. Nothing much can 



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Fig. 3. — Diagram showing the average parts per million of water-soluble nitrates in dry soil to a depth 
of 2 feet in the siininier-fallow, spring-wheat, and corn plats; also weekly increase of wheat plants 
and precipitation curve. 

])e said about this curve beyond the statement that the rapid decrease 
in nitrates corresponds quite closely to the period of most rapid 
growth of the corn as recorded in the notes at the time. 

The corn plat had evidently accumulated quite large amounts of 
nitrates before the ])lants were large enough to materially draw upon 
these salts, for the corn curve shows that on June 19 the accumula- 
tion of nitrates in the corn plat had reached about the same point as 
that in the fallow ])lat. The direction of the corn-plat curve from 
June 19 to June 24 also suggests the general drop found in the fallow 
and wlieat plat curves. As the corn was not planted imtil May 7, the 
plat was evidently in a state of summer fallow until determinations 
were begun. During the most active growing period of the corn 
these plants evidently reduced tlie amounts of nitrates to as low a 

173 



DISCUSSION OF RESULTS OF DETERMINATIONS. 



17 



content as did the wheat plants in the wheat plat. Apparently the 
corn plants were as fond of the nitrates as were the wheat plants. 
The general field observation that a corn crop renders the ground 
more productive for grain than a previous grain crop is apparently 
not due to the leaving of greater amounts of nitrates in the soil. 

WITHDRAWAL OF NITRATES BY CORN ROOTS, 

As already mentioned, the general decrease in the amounts of 
water-soluble nitrates in ])oth the fallow and the wheat plats masked 
the effect of any progressive withdrawal of the nitrates by the wheat- 
plant roots in the successive soil layers. In the case of the corn plat, 
however, there was the advantage of the fact that the nitrates had 
already been accumulated, and so the corn offered an opportunity for 
observing]: the removal of nitrates from the various soil lavers as the 



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JULY 1 



Fig. 4.— Diagram showing the parts per million of \vater-soIul)!e nitrates in dry soil in each li-iueh soil 
layer in the corn plat to a depth of 2 feet. 

season progressed. Figure 4 shows the probable effect of the feed- 
ing of the corn roots and indicates that the surface 6-incli layer early 
lost considerable amounts of nitrates and that very shortly there- 
after the 12-inch layer was drawn upon most heavily. From July 3 
to July 17 the corn plants made their most rapid growth, and the 
curves also show that at this period the nitrates were most heavily 
drawn upon. The 18-inch layer' contained most nitrates and lost 
most. All of the layers to a depth of 2 feet were probably reduced 
to the limit of the powers of the corn roots to withdraw nitrates from 
this heavy soil. 

The curves in figure 4 also show variations in the activity of 
nitrification, especially an increase from June 24 to July 3, but the 

173 



18 SEASONAL KITRTFICATION AS INFLUENCED BY CROPS, ETC. 

progressive downward feeding of the corn roots seems clearly shown. 
The characteristic manner of surface feeding of corn roots would early 
draw upon the plant food in the surface soil, as indicated in figure 4 
by the curves of the 6 and 12 inch layers. The soil samples, however, 
were taken not between the corn rows, where the lateral roots only 
were abundant, but as near the corn plant as they could be taken, 
not more than 3 or 4 inches away. Such sampling would encounter 
the primary roots of the corn, as well as penetrate the soil area in 
which the lateral roots were feeding most actively, and it seems 
probable that the reduction of the amounts of nitrates in the 18 and 
24 inch layers was due to the penetration of these primary roots to 
that depth. This would account for the tardy removal at the lower 
depth, as the primary roots would be longer in reaching these lower 
layers. The moisture determinations showed that the moisture was 
being removed quite rapidly from the 18 and 24 inch soil layers at 
the same time. 

The most striking feature about figure 3 is the general parallelism 
of the nitrate curve of the wheat plat with the nitrate curve of the 
fallow plat. The seven-day lagging of the maxima in the wheat plat 
is not, however, as clearly shown here as in figures 1 and 2, but is 
evident in the early part of the season. The only other point of 
difference in the two curves is in the period from May 29 to June 12, 
during which time the summer-fallow plat steadily accumulated 
nitrates, while there was a bare increase in these salts in the wheat 
plat. This is the period during which the fallow plat laid in its 
increased stock of water-soluble nitrates, and from June 12 on there 
is a constant difference of about twenty to thirty-five parts per 
million in favor of fallowing. It is probable that this difference 
represents in general the continuous amount of soluble nitrates used 
up b}" the plants. It is, however, also likely that had more nitrates 
been formed the plants woidd have used them and that the curves 
represent in a general way the minimum amount to which the corn 
and wheat roots can reduce the nitrates in this heavy soil. Traps ^ 
found that the nitrogen content of crops was increased with an 
increased nitrifying activity in the soil. 

In figure 3 no close relation is evident between the weekly crop- 
increase curve and the decrease in the nitrates, using the fallow curve 
as a check. 

RELATION OF NITRIFICATION TO FIELD FACTORS. 

The nitrification in the soil depending, as it does, on so many 
factors — temperature, moisture, aeration, organic matter, kind of 
croj), cultivation, etc. — it is difficult in the field to run down the 



a Traps, G. S. Bulletin 106, Texas Agricultural Experiment Station, p. 4. 
173 



DISCUSSION OF RESULTS OF DETERMINATIONS. 



19 



specific pliysical or chemical factors most influential in the promoting 
of the formation of nitrates in a given soil type. An attempt was 
made to correlate the variations in and tho amounts of the nitrates 
found during the season with some of the factors known to influence 
their formation. As some of the most important ones from a 
chemical ov bacteriological standpoint could not be taken up at the 
time the attempt at correlation had to be confined to a consideration 
of some of the physical factors. As, however, the soil of the diflerent 
plats was similar and probably as nearly uniform as it is possible to 
obtain soil in different plats, and, furthermore, as the j^ast histor}' of 
the plats was exactly the same up to the time of beginning the work, 
it is not likely that there were any fundamental chemical or bacteri- 
ological differences in the plats at the time of planting. The wheat 
and corn plats were adjacent and were separated from the fallow plat 
by only 6 rods, and certainly no differences were apparent. Continu- 
ous air-temperature records were kept by means of a thermograph, 



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Fig. 5. — Diagi'am showing' tlie average parts per million of water-soluble nitrates in dry soil in the fallow 
plat to a depth of 2 feet: also soil-moisture curve and precipitation cuive. 

but no evident relation could be obtained between the variations in 
amounts of nitrates and in temperature, whether the mean maximum, 
mean minimum, or general mean temperature was considered. 

Figure 5 shows the curve of average amounts of nitrates to a depth 
of 2 feet in the f allow" plat, the average soil-moi'sture curve to the 
same depth, and the precipitation curve. It is evident that neither 
the soil-moistm-e curve nor the precipitation curve shows any close 
agreement with the nitrate curve. The moisture curve is nearly 
straight, varying onh^ from 24 to 29 per cent during the whole season. 
The other two curves i)arallel from June 19 to the end of the season, 
but as the formation of nitrates had already taken place and as the 
amounts thereafter remained constant, it is likely' that this agreement 
is only incidental. It might be mentioned, incidentally, that the 

173 



20 SEASONAL NITRIFICATION AS INFLUENCED BY CROPS, ETC. 

mean temperature of the air did not reach nearly its maximum until 
some time later than June 19; in fact, the maximum temperature 
occurred in September. 

In figure 6, which gives the corresponding data obtained on the 
wheat plat, there is a closer relation between the moisture and the 
nitrate curves, but as the growing crop lias removed much of the 
moisture and the nitrates from the soil it is apparent that no close 
causal relationship could be insisted on in this case either. There 
is, on the other hand, a close agreement between the moisture curve 
and the weekly crop-increase curve. The hygroscopic coefficient for 
this soil is very high (about 12 per cent), and judging from the gen- 
eral appearance of the crop and of the soil at the time it is likely 
that the moisture curve from the first part of July represents about 
the minimum to which the wheat plants could reduce the moisture 
content of the soil, and the same holds true in regard to the nitrate 



































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Fig. 6. — Diagram showing tlie average parts per million of water-soluble nitrates in dry soil to a depth of 
2 feet in the spring-wheat plat; also soil-moisture, precipitation, and weekly crop-increase curves. 

curve, as mentioned earlier. This same removal of moisture would, 
of course, also check nitrification, so both this and the plant feeding 
would keep the nitrates from increasing. 

Figure 7 brings out more clearly than figures 1 and 2 the phenome- 
non of the general parallelism in the maximum nitrate accumulation 
in the individual soil layers in the wheat and fallow plats. This 
figure does not show the absolute amounts of nitrates found in the 
soil, but shows simply the period during which the respective 6-inch 
soil layers in the fallow and wheat plats contained an amount of 
nitrates greater than any other 6-inch layer. It will be noticed that 
there is a very close ag;"eement between the two plats in the depth 

173 



DISCUSSION OF RESULTS OF DETERMINATIONS. 



21 



at which nitrates were most activeh' accumulating at different 
periods and that other factors than cropping evidently determine 
the seasonal activity of nitrification. 



THE DISAPPEARANCE OF NITRATES. 

Glancing again at figures 1 and '_' it wt)uld naturally be asked what 
became of the water-soluble nitrates in the individual 6-inch soil 
layers after accumulation had reached a maximum, and why the 
seasonal shifting of the maximum amount of nitrates has such a dis- 
tinct downward movement. It would be expected that the rainfall 
would tend to wash down the nitrates along with other salts, and 
such probably did take place to some extent. Selecting the week 
from May 9 to IMay 16, in figure 2, there would seem to have been 









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Fig. ".—Diagram showing tlie time aud location of the maximum amount of nitrates in the wheat and 

fallow plats. 

such a movement from the surface layer to the 12-inch layer. The 
rainfall for that week was 1.25 inches. This is the only instance, 
however, in the two figures which shows the plausibility of such an 
explanation of the apparent downward "movement" of the nitrates. 
Inspecting figure 1, where no growing crop complicates the results, 
it would even appear that the rainfall had the opposite effect, as, for 
instance, during the period from May 22 to May 29. The effect seen 
there could not be explained by the washing down of the nitrates by 
the rains, as there was an increase in nitrates in all of the soil la3"ers 

173 



22 SEASONAL NITRIFICATION AS INFLUENCED BY CROPS, ETC. 

except the 12-inch hiyer during that week. The same point is 
brought out (hiring the period from June 5 to June 12, there being 
no rainfall at all to wash down the nitrates from the surface into the 
12, 18, and 24 inch layers. And, finally, no such explanation can 
be given for the sudden drop in amounts of nitrates during the 
period from June 12 to June 19, when there was practically no rain. 

DENITRIFICATION . 

At the beginning of the work denitrification was expected to 
become an obvious factor in affecting the seasonal changes of nitrates 
in the soil, and ammonification was also expected to become of 
measurable importance; accordingly, determinations of nitrites and 
ammonia were made. Neither of these activities, however, assumed 
any importance at all during any part of the season so far as the 
analyses brought out, and, accordingly, toward the end of the season 
ammonia and nitrite determinations were not regularly made — 
sufficiently often, however (usually every other week), to know the 
amounts present of these compounds. 

No conveniences were at hand for making ammonia determina- 
tions directly on the fresh soil, and this is obviously the best way of 
determining these compounds. The determination of the water- 
soluble ammonium compounds could not be expected to give reliable 
data. That ammonia ma}^ have been formed in larger amounts than 
would be suggested by the quantities found during the season would 
appear from the work of Stevens and Withers,'^ who state that in 
some soils "nitrification proceeds as rapidly as ammonification, con- 
verting the ammonia to nitrate as fast as it is rendered available by 
the ammonifying organisms.'' 

The amounts of nitrites found varied during the whole season from 
nothing to 1.8 parts per million in the dry soil. There was no regu- 
larity in even these small amounts, one week traces only or none at 
all being found and the next week traces to 1.8 parts per million. 
The same irregularities held true in the individual soil layers. The 
usual amounts of nitrites were from 0.1 to 0.3 part per million, and 
one single sample — from the 6-inch layer in the wheat plat, on May 
22 — had 6 parts per milhon, the only nitrite determination made 
thafexceedetl 1 .8 parts per million at any time during the season. 

The determinations likewise showed the presence of but small 
quantities of ammonia per million parts of dry soil, but in this case 
there was a slight decrease toward the end of the season. During the 
earlier part of the season, the amounts found averaged 6 to 8 parts 

a Stevens, F. L., Withers, W. A., et al. Centralblatt fiir Bakteriologie, Parasiten- 
kunde imd Infektionskrankheiten, pt. 2, 1909, p. 23; abstracted in Experiment Sta- 
tion Record, vol. 21, no. 2, 1909, pp. 118-119. 
173 



DISCUS'SION OF RESULTS OF DETERMINATIONS. 23 

of ammonia per iiiillion ami toward tlie end of the season the amounts 
fell to about 3.6 to 4.7 parts per million. No such gradual decrease 
in amounts of nitrites toward the end of the season occurred.. The 
variations in the quantities of ammonia and of nitrites in the indi- 
vidual soil layers were apparently not then sufficient to account for 
the marked and general and consistent changes obseived in the case 
of the nitrates. 

It may be remarked that although the soil is heavy in texture and 
likely to compact under bad treatment, the soil on the ])lats was (|uite 
loose and open, it being the first year after breaking. Also, toward 
the middle of summer the soils cracked quite freely, so that there was 
fair opportunity for soil aeration all season, and, as the precipitation 
curve will show, there was no trouble with water-logging of the soil. 
All conditions considered, it is apparent that conditions were not 
extremely favorable for active denitrification. 

SEASONAL VARIATIONS IN NITRIFICATION. 

Referring again briefly to all of the figures, one is struck by the 
remarkable uniformity of direction of all the curves, especially in 
figures 1, 2, and 3. The facts that the same general variations exist 
in all cases and that denitrification is probably not responsible for 
the nitrate changes point to a general seasonal phenomenon which is 
affected in degree but not in kind by the presence of the growing 
crops. 

It must be kept in mmd that the nitrates found w'ere those soluble 
in water and that the curves thus represent only the water-soluble 
nitrates. It would appear that nitrification began earliest in the 
surface layer of the soil and that the maximum activity of this proc- 
ess gradually extended downward into the successively lower 6-inch 
layers and reached its maximum in any one layer in general at a 
period of about two weeks later than in the next 6-inch layer above. 
In other words, conditions of temperature, aeration, etc., were favor- 
able for nitrification in the surface layer earliest, as would of course 
be expected, and these favorable conditions obtained later in the 
lower soil layers. By June 12 these requisite conditions had in 
general reached their optimum throughout the 2-foot soil section 
studied, and it might well be conceived that the physiological activity 
of the bacterial flora necessary for nitrification reached its maximum 
in the comparatively short period of about two or three w^eeks in each 
soil layer after the conditions had once become most favorable and 
that the general ''crop" of nitrifying bacteria matured by the middle 
of June. 

Under this interpretation it would be inferred that the nitrif3ang 
bacteria had an active growing season, so to speak, of two to three 
173 



24 SEASONAL ISLITRIFICATION AS INFLUEN(!ED BY CROPS, ETC. 

weeks after favorable climatic conditions had been established and 
that after this period of growth they underwent a resting period, or 
at least their physiological activity was niuch reduced, just as is the 
case with an annual flora of higher plants. From this explanation 
one would expect that their general activity would be more intense 
and begin earlier in the fallow plat than in the wheat plat, as the 
fornier was spring-})lowed to a good depth, while the latter was not 
so treated — and this is the case. As already noted, the maximum 
nitrate content in the respective soil layers occurred in genera] about 
one week earlier in the fallow than in the wheat plat, but the general 
"maturity of the crop," to continue the analogy of the higher flora, 
was effected at about the same time in the two instances, viz, about 
June 19. This seasonal deciease in nitrification may also be caused 
by a decrease in food readily available for the nitrifying organisms, 
^^^lether the seasonal variation in amounts of nitrates is due to 
periodicity of the activity of the nitrifying organisms in the soil, 
unaffected by any climatic or food factor, or whether it is due to 
lack of sufficient food for continued activity would be hard to decide 
from the various curves. As somewhat similar variations may be 
obtained in the laboratory under controlled food conditions, this 
factor must be kept in mind. Sclmeider'* found that the fixation of 
nitrogen in the soil was greatly increased by the addition of organic 
food, as also by the addition of potassium phosphate and of lime; 
and Bazarewski ^ found that the addition of small amoimts of organic 
food promoted nitrification, but that the addition of larger amounts 
decreased the activity of the organisms. 

Notwithstanding the fact that neither nitrites nor ammonia were 
ever found in sufficient amounts to account for the actual decrease 
in amounts of nitrates, other organisms active in the production of 
these compounds may conceivably have increased vegetatively at the 
expense of the nitrates alread}' ])resent. Whether the food supply 
itself could be entirely responsible for the variations in the amounts 
of nitrates it would be hard to decide, but certainl}- the prairie soil in 
question is considered quite fertile so far as the ordinary farm crops 
are concerned, and the j)lats on which these determinations were 
n*ade had never been cropped before. 

That this rather definite and comparatively short seasonal period of 
activity of nitrification has not been limited by lack of moisture in 
the soil woidd seem to be indicated by the moisture conditions in the 
summer-fallow jdat. The drying up of the soil in the wheat plat, due 
to the demands made upon the moisture by the plants, would of 

o Schneider, P. Landwirtschaftliche Jahrbiicher, vol. 35, sup. 4, 1906, pp. 63-83; 
abstract in Experiment Station Record, vol. 18, no. 8, April, 1907, p. 722. 

b Bazarewski, L. von. Nitrification and Denitrification in Soils. Reviewed 
in Experiment Station Record, vol. 21, no. 1, July, 1909, pp. 20-21. 
173 



DISCUSSION OF RESULTS OF DETERMINATIONS, 25 

itself inhibit nitrification, just as excessive dryness would check the 
growth of any other plant. 

It seems evident, therefore, that the decrease in amounts of nitrates 
can not be wholly explained by either moisture content of the soil, 
denitrification, animonification, or temperature. A shortage in or- 
ganic food supply may have been one of the principal causes, but it 
is also possible that the amounts of nitrates accumulated by the 
nitrifying organisms would inhibit further increase in these salts. 
Instances of such inhibition are not uncommon in the case of other 
bacteria and fungi. It is also within the province of possibility that 
the nitrifying organisms may have accumulated compounds as by- 
products other than nitrates and that these compounds might have 
checked any otherwise possible increase in amounts of nitrates. Even 
when all these factors are considered there would appear to be a 
rhythmic periodicity of nitrifying activity during the season due to 
some unknown property of the bacteria. 

CHANGE OF WATER-SOLUBLE NITRATES TO ALBUIVIINOID NITROGEN, 

The disappearance of the water-soluble nitrates does not by any 
means indicate that the nitrogen was actually lost from the soil, but 
that it was perhaps converted into albuminoid nitrogen by other 
groups of bacteria. It is quite conceivable that the food require- 
ments might be sufficiently different for the two groups to permit a 
considerable decrease in the activity of one group and at the same 
time increase the activity of another group. It may very well be that 
the artificial application of a food supply suitable for use by the 
nitrifying organisms would again increase their activity to a point 
beyond that which they formerly had reached. It is likely that had 
water-insoluble nitrogen been determined in the same soil series, no 
general decrease would have been found in this from June 12 to June 
19; on the contrary, there would probably have been an increase. 
In this connection it may be mentioned that Montanari " found least 
amounts of "nitric nitrogen" in February, March, and June, and 
most in July and August. He found that plats growing wheat, rape, 
and corn contained more nitrates in the lower stratum (10 to 20 centi- 
meters) than in the upper (0 to 10 centimeters); on a plat growing 
Medicago, however, this relationship was reversed. This accumulation 
of the (probably) albuminoid nitrogen would very possibly be of use 
to plants the following spring and would simply be held in reserve in 
the soil and either break down into readily water-soluble forms or in 
other ways become available for use by higher plants. Whitson, 

a Montanari, C. Quantity of Nitric Nitrogen in Soils Variously Cultivated. Le 
Stazioni Sperimentali Agrari Italiane, vol. 41, 1908; abstract in Experiment Station 
Record, vol. 20, no. 8, March, 1909. 
173 



26 SEASONAL NITRIFICATION AS INFLUENCED BY CROPS, ETC. 

Stoddart, and McLeod'^ found that the difference in the amount of 
nitrogen between virgin hind and cropped hind was not enough to 
account for the crops removed, and they suggest that this fact is due 
to the removal of nitrogen by bacteria. Their work was carried on in 
humid regions, however, where leaching would affect the distribution 
of nitrogen in the soil. 

In an area such as the prairie region represented, where the amount 
of rainfall is rather limited, there would be an accumulation in the 
fallow plat of both water-soluble and albuminoid nitrogen. The crop 
would have the advantage of the nutritive condition of the soil solu- 
tion represented on July 31 in figure 1, as compared with the less 
favorable conditions obtaining on April 27 in figure 2. 

It should be kept in mind that the plats on which the determina- 
tions were made were ''dry farmed," and it is not intended strictly to 
apply to irrigated lands the conclusions derived from this work. 
Irrigation would in all probability cause disturbances in the bacterial 
processes in the soil, and so, to some extent at least, mask the seasonal 
activities of the bacterial flora by translocation of the nitrates and by 
intermittent wetting and drying. It is likely, however, that by 
properly controlling the work the same general phenomena would be 
observed. 

RELATION OF NITRIFICATION TO SUMMER FALLOW. 

As will be noticed from figure 3, the summer-fallow plat contained 
on July 31 about 35 parts per million more of water-soluble nitrates 
than did the wheat plat, which would present a more favorable nutri- 
tive condition than found in the cropped plat. In view of the fact, 
however, that the fundamental phenomena underlying nitrification 
are the same in both the cropped plats and the fallow plat, it would 
seem that summer fallow would be of doubtful economic value so far 
as the use of nitrates is concerned. The various curves distinctly 
show that nitrification is affected in degree but not in kind by the 
presence of a growing crop, and it is certainly likely, with this point 
in view, that nitjates were formed to as large an extent in the wheat 
plat as in the summer-fallow plat; interest on the investment would 
be lost from the fallow land while lying idle, when by suitable treat- 
ment of the soil use could as well be made of the products of the 
nitrifying organisms each year as to allow the land to lie idle. The 
bacterial groups active in producing soluble nitrates and albuminoid 
nitrogen, feeding largely on the organic matter in the soil, would of 
course reduce their food supply more rapidly in a system of fallowing 
than in a system of crop rotation, where organic matter is continually 
being added to the soil. Another serious objection to summer fal- 

aWhitson, A. R., Stoddart, W. C, and McLeod, A. F. Twenty-Third Annual 
Report, Agricultural Experiment Station of the University of Wisconsin, 1906, pp. 
160-170. 
178 



SUMMARY. 27 

lowing;, aside from the one just mentioned, is the fact that favoial)le 
conditions for chemical oxidation of the organic matter obtain because 
of the more ready access of heat to the soil due to its exposed condi- 
tion. So in an area such as is represented by this station where even 
so far as moisture is concerned fallowing is questionable as a system, 
the nitrifying flora would be served bettor l)y eliminating fallowing. 

SUMMARY. 

In the early part of spring the surface 6 inches of soil contained 
more water-soluble nitrates than the lower 6-inch layers, and the 
amount reached its maximum in the first layer in the first and second 
weeks in May. 

From that time the amounts of water-soluble nitrates decreased in 
the surface layer, and at the same time the nitrates in the 12-inch 
layer increased rapidly for one to two weeks, when a decrease was in 
turn shown in that layer; similarly with the 18-incli and 24-inch 
layers. 

Thus the successive 6-inch layers from the surface 6-inch layer 
downward, at periods of about two weeks apart, each contained more 
water-soluble nitrates than any other layer at the same date, and 
each layer after accumulating a maximum amount showed a decided 
decrease, which it never again made up. 

An exception w^as the 24-inch layer, which, while it showed the 
same phenomenon as distinctly, did not at any time accumulate 
more nitrates than any other layer, but underwent the same general 
changes as the upper layers. 

The respective soil layers in the fallow plat reached their indi- 
vidual maximum accumulation of nitrates about a week earlier than 
the corresponding layers in the wheat plat. From June 12 to June 
19 all soil layers in both plats suffered a general decrease in amounts 
of water-soluble nitrates. This occurrence took place in the spring- 
wheat plat in the same order and in about the same magnitude as in 
the fallow plat. 

No evidence could be brought out that the wheat-plant roots 
drew more heavily on the water-soluble nitrates in one 6-inch soil 
layer than in any other at any time. The general decrease of nitrates 
in all of the soil layers in the fallow plat masked any such reduction 
by the wheat if any occurred. 

The corn plants at the end of their most active season of growth 
had reduced the amounts of water-soluble nitrates in the soil to the 
same degree of exhaustion as had the wheat plants in the correspond- 
ing period of their development. The roots of the corn plants prob- 
ably used" the nitrates in the surface soil layer first, then drew most 
heavily upon the 12-inch layer, and finally upon the 18 and 24 inch 
layers. This progressive downward feeding of the corn roots occupied 

173 



28 SEASONAL NITRIFICATION AS INFLUENCED BY CROPS, ETC. 

the time from June 19 to July 17; at the latter date the nitrates had 
probably been reduced by the corn roots to the lowest limit possible. 

The wheat and corn plants reduced the nitrates to a constant 
amount — about 15 parts per milHon in the dry soil. The wheat 
removed the moisture also to a fairly constant degree, namely, about 
15 per cent. 

Denitrification, as measured by the nitrites found, could not 
account for the changes occurring in the nitrate content in the soil 
layers. 

No correlation could be established between the amounts of nitrates 
and the air temperature or between the nitrates and the soil moisture. 

Translocation of the nitrates by rain could not account for the 
seasonal changes in the amounts of nitrates in the different soil layers. 

The general increase and decrease in water-soluble nitrates followed 
the same course in the fallow plat and in the cropped plats, the only 
''general difl'erence being that there were more nitrates in the fallow 
plat after May 16 than in the wheat plat. 

It is suggested: 

(1) That the marked increase in the amounts of water-soluble 
nitrates in the individual soil layers, followed by marked decrease 
in the same, and not at any time again reaching the same quantity, 
is due to a seasonal physiological activity of the bacterial flora, 
depending upon the climatic and nutritive factors in the soil, or else 
the seasonal changes are due to a rythmic periodicity of activity in 
the nitrif^nng organisms during the season, the nature of which 
could not be made clear by the data obtained. 

(2) That the general decrease in amounts of water-soluble nitrates 
from June 12 to June 19 may be due to the formation of albuminoid 
nitrogen by other groups of bacteria which may have become most 
active at this time; that the excessive accumulation of nitrates or 
the accumulation of other by-products by the nitrifying organisms 
may have inhibited further activity on their part. 

(3) That, while the summer-fallow plat contained at the end of the 
season more water-soluble nitrates than did the corn and wheat 
plats, the same general phenomena of nitrification are found to take 
place in both fallow and cropped plats ; hence the extended practice 
of summer-fallowing would seem to be of doubtful value so far as the 
accumulation of nitrates in the soils represented by this station is 
concerned. 

Note. — Since this bulletin was put into type, a review of Kansas Station Bulletin 
161, appearing in the Experiment Station Record, vol. 22, no. 1, p. 21, gives as one 
of the conclusions arrived at by the authors, that "bacterial life and activity seem 
to rise and fall with more or less regularity. These periods of maximum and mini- 
mum activity are to a certain extent independent of moisture and temperature, and 
are possibly due to the presence of bacterial by-products." This conclusion corre- 
sponds closely with the work herein descril)ed and shows that other bacterial groups 
besides the nitrifying flora have the periodicity mentioned in this bulletin. 
173 



INDEX, 



Page. 

Aeration, soil of experimental ]ilats 23 

Agropyron occidentale, occurrence in sod of experimental plats 9 

Alkali salts. See Salts, alkali. 

Ammonia in dry soil treated in various ways 11, 22-23 

nitrites, and nitrates in soil, extraction, results of various nielhods. ... 11 

Ammonification, relation to variation of nitrates 22-23 

Bacteria, behavior in soil 28 

nitrifying, growing season , 23-24 

Bazarewski, L. von, nitrification in soil, remarks 13, 24 

Bellefourche, S. Dak., inauguration of experiments in dry-land agriculture 8 

Bouteloua oligostachya, occurrence in sod of experimental plats 9 

Buffalo grass, occurrence in sod of experimental plats 9 

Bulbilis dactyloides, occurrence in sod of experimental plats 9 

Carex filifolia, occurrence in sod of experimental plats 9 

stenophylla, occurrence in sod of experimental plats 9 

Color, foreign organic, in soil, cause of error in cletermination of nitrates 10 

Corn, nitrates present in plat, comparison with spring- wheat and fallow plats 16-17 

roots, withdrawal of nitrates 17-18 

Crops, nitrogen content, increase with increased nitrifying activity in soil 18 

Denitrification, relation to variation of nitrates, etc 22-23 

Dry-land experiments. See Experiments, dry-land. 

Experiments, dry-land, field history of jjlats 9 

methods of making chemical determinations 9-11 

nitrates in fallow plat 12-14 

results of chemical determinations 12-27 

Fallow plat, comparative variation of moisture, precipitation, and nitrates. . . . 19-20 
nitrates present, comparison with spring-wheat and corn i)lats... ](i-17 

removal, comparison with wheat plat 15 

variation 1 2-14 

summer, objections 26-27 

relation of iiitrification 26-27 

Fixation, nitrogen, statement of P. Schneider 24 

Food, organic, for nitrifying organisms, effect on nitrogen content of soil 24 

Grama grass, occurrence in sod of experimental plats 9 

Grass, buffalo, occurrence in sod of experimental plats 9 

grama, occurrence in sod of experimental plats 9 

Grasses, varieties in sod of experimental plats 9 

"Gumbo" land. See Land, "gumbo." 

Hunt, T. F., statement on depth of nitrification in soil 13 

Introduction to bulletin 7-8 

Land, "gumbo," experiments in handling for crops 8-28 

McLeod, A. F., Whitson, A. R., and Stoddart, W. 0., statement concerning 
nitrogen content of soil 25-26 

173 

29 



30 SEASONAL NITRIFICATION AS INFLUENCED BY CROPS, ETC. 

Page. 
Matter, soluble organic, in soil, cause of error in determination of nitrates. ... 10 
Moisture, precipitation, and nitrates in experimental plats, comparative varia- 
tion -. 19-20 

Montanari, C, remarks on quantity of "nitric nitrogen " in soil 25 

Nitrates, determination, three sources of error 10 

in dry soil treated in various ways 11 

experimental plats, disappearance 21-22 

fallow, spring-wheat, and corn plats, comparison 16-17 

soil, decrease, causes 25 

spring-wheat plat, variation 14-16 

wheat plat, removal, comparison with fallow plat 15 

moisture, and precipitation in experimental plats, comparative varia- 
tion 19-20 

nitrites, and ammonia in soil, extraction, results of various methods. . 11 

variations in plats of different crops 12-27 

water-soluble, change to albuminoid nitrogen 25-26 

disappearance 25-26 

withdrawal by corn roots 17-18 

Nitrification, factors causing check 20 

maximum, period required for development 23-24 

relation to field factors 18-21 

summer fallow 26-27 

seasonal variations 23-25 

soil, effect of plowing 15 

relation of moisture 24-25 

remarks of L. von Bazarewski 24 

Nitrites in dry soil treated in various ways 11, 22-23 

nitrates, and ammonia in soil, extraction, results of various methods 11 

Nitrogen, albuminoid, change from water-soluble nitrates 25 

content, crops, increase with increased nitrifying activity in soil 18 

soil, effect of organic food for nitrifying organisms 24 

virgin and cropped 25-26 

fixation, statement of P. Schneider 24 

nitric, quantity in soil, remarks of C. Montanari 25 

Organic matter. Sec Matter. 

Plats, experimental, descrii)ti()n 8 

field history 9 

Plowing, effect on nitrification in soil 15 

method on experimental plat 9 

Precipitation, moisture, and nitrates in oxi)erimental jtlats, comparative 

variation 19-20 

Prairie, handling lands, establishment of experiment stations 7-8 

sod, experimental plats, description 9 

Roots, corn, withdrawal of nitrates 17-18 

Salts, alkali, presence in soil a cause of error in determination of nitrates 10 

Schneider, P., nitrogen fixation, statement 24 

Sedges, occurrence in sod of experimental plat 9 

Soil, behavior of bacteria 28 

dry, amounts of nitrites and ammonia found 22-23 

experimental plats, aeration 23 

cracks 23 

descrijjtion 8 

173 



INDEX. 31 

Page. 

Soil, "gumbo, " depth of moisture, comparison of broken and unbroken soil 13-14 

description 8 

nitrates, causes of decrease 25 

nitrification, effect of plowing 15 

relation of moisture 24-25 

to field factors 18-21, 

remarks of L. von Bazarewski 24 

nitrogen content, effect of organic food for nitrifying organisms 24 

virgin and cropped, nitrogen content 25-26 

South Dakota, Bellefourche, inauguration of experiments in dry-land agricul- 
ture 8 

Spring wheat. See Wheat, spring. 

Stevens, F. L., and Withers, W. A., statement concerning nitrification 22 

Stoddart, W. C, Whitson, A. R., and McLeod, A. F., statement concerning 

nitrogen content of soil 25-26 

Summary of bulletin 27-28 

Summer fallow. See Fallow, summer. 

Thermograph, use in recording air temperature 19 

Traps, G. S., remarks on relation of nitrates in crops to nitrifying activity in 

soil 18 

Vetches, occurrence in sod of experimental plats 9 

"WTieat-grass, occurrence in sod of experimental plats 9 

plat, removal of nitrates, comparison with fallow plat 15 

spring, plat, nitrates present, comparison with fallow and corn jjlats . . . 16-17 

variation 14-16 

Whitson, A. R., Stoddart, W. C, and McLeod, A. F., statement concerning 

nitrogen content of soil 25-26 

Withers, W. A., and Stevens, F. L., statement concerning nitrification 22 

173 

o 



[Continued from page 2 of cover.] 

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